This is a conceptual design study for a heavy explorer class of starships. This class is based on ideas proposed by the group and a series of driving assumptions. The resulting craft is a large, capable, mobile research platform with enough personnel and resources to adequately explore a star system. It is also rather large, slow, and expensive. Hopefully this can be used as a draft design for group debate and refinement.

No unforseen advances in technology. This is probably the most unreasonable assumption of the group, but I will assume no fantastic new technologies that are not currently well along in some form of research and development.

A fairly extensive space exploration and industrial infrastructure is already in place. If we are not well into exploring and exploiting this solar system. We probably are not going to be interested in checking out the neighboring ones. And of course if we are not doing big projects in space, we will not have the industrial capacity to building and launch this ship.

The target star system was extensively surveyed by high resolution orbital telescopes in the Sol system. With synthetic aperture orbital telescopes in the Sol system we should be able to get high resolution photos of the planets in the target star system. This means the exploration crew will have a fairly detailed idea of the nature of all the target planets. This seems a reasonable assumption given the current plans and proposals for space telescopes.

The public is interested in a serious exploration of the target starsystem. Not merely an Apollo style "plant the flag and return" mission. This requires a fairly large crew (at least in the hundreds) with extensive support equipment. Which requires a large ship and support crew.

Small automated "Pathfinder" ships were developed and launched ahead of the main ship to warn it of any dangerous debris, and survey the interstellar media. As will be discussed elsewhere, our current lack of basic information on what's in interstellar space, and in what concentrations, fundamentally limits our discussion of drive systems.

The ship is capable of a speed of at least 30% of light speed (0.3c). This isn't close enough to lightspeed to take serious advantage of relativistic time dilation. This means the ship and crew will experience, and must function for, a round trip flight time of up to twenty five to thirty years for even the closest of target star systems. This is nearly the upper limit of what can be expected of the ship's systems (its the typical service life of most naval vessels and large ground systems like power plants), and is probably at or beyond the ability of most potential crew.

Spin Gravity. The crew can't remain healthy over the long term without artificial gravity producing one gee, and being spun at a rate of 3 revolutions per minute or less. This requires any rotating habitation area to be at least 200 meters across, and will strongly define the dimensions of the ship.

No multi-generation crews. This idea seems popular among some members of the group and some science fiction authors. However, it greatly weakens the concept of an interstellar expedition both from a crew standpoint and a engineering standpoint.

From a crew standpoint it is far harder to attract top personnel. A top person might be willing to devote their entire professional life to exploring a new star system. But if they will never reach the star system, merely start the flight, they'll be much less eager to sign up. Also there will be far less interest (read support) back home for a project that won't get anywhere for generations. Why send a slow ship out now when you can certainly make a faster one later.

Also, frankly, from an engineering standpoint a multi-generation ship causes far more problems than it solves. The crew must be made at least a few times larger since most of them won't be in the professional part of their lives. Which makes the ship larger and more expensive. On top of that, nothing as complicated as a starship has ever been designed and run for more than a couple decades without major overhauls. To build something that can last indefinitely, it would need practically all the equipment onboard to continuously rebuild itself from scratch, with the appropriate increase in crew size to run all that. You wind up moving from designing a ship to designing a multimillion person mobile space colony. All in all, its a lot more practical, and simpler, to just make a faster ship.

Multi-cycle RAIR - This system was the original one I suggested for the Explorer class starship design. It used a modified ramscoop that would scoop up fuel launched by a launchers orbiting in our solar system, and scoop up interstellar reaction mass. Unfortunately we don't know what's in interstellar space to scoop up, and current assumptions would suggest that there is to little to bother with.

Externally Fueled Fusion - This system was the final and current one I suggested for the Explorer class starship design. Launchers orbiting in our solar system launch fuel to it to power it during its acceleration out of our solar system, and it would need to carry enough fuel to decelerate it into the target starsystem.

The Externally fueled Fusion rocket concept is a solid baseline design. It violates no rules of physics, and doesn't require any dramatic new technologies to work. So for the rest of the discussion of this Explorer Class of starships we will assume it is the drive system in use. However this system would require the construction of a massive space based fuel launcher and the mining of incredible amounts of fusion reactor fuel, so it would be very expensive.

More important to the assumptions of the LIT group, this drive system is not capable of near relativistic flight. It probably can't get to any faster then 1/4th to 1/3rd of light speed. That means that it isn't suitable for a mission to Tau Ceti (the reference mission under study by L.I.T.) since it would take 30 to 40 years to get there. Which is far to long for a ship as 'small' as an Explorer class, and probably far to long for practical use.

As you probably already gathered. This ship will need a lot of fuel to accelerate. Oh, for the purpose of this illustration I'm going to create a constant called a Ship_g. A normal g is 9.8 m/s^2. (Meters per second, per second) I like round numbers, so Explorer class ships will accelerate at 10 m/s^2. The crew and ship won't care much, but it makes for much better numbers.

Assuming we're go to accelerate to 1/3rd of light speed (100 million meters per second, a.k.a. 1. E8 m/s). At 1 Ship_g (10m/s^2) our ship will take. E7 seconds to get to speed. That's 115.7 days or about 16.5 weeks. All that time its out accelerating a Corvette (The Explorer class starship has a 0-60mph time of 2.7 seconds.). All that time the home fuel launchers are sending us care packets of fuel. How much fuel?

To accelerate a 25,000,000 ton loaded ship at 10m/s, with a fusion engine with a specific impulse of 2,500,000 we will need to 'burn' about 10 TONS! of fusion fuel per second! You'll need 100,000,000 tons (that's a hundred million tons!) of fuel to accelerate it up to 1/3rd light speed. Frankly, I don't know where we could find that much!

Ship dry weight total

500,000 tons

Deceleration Fuel mass (50 to 1 ratio)

25,000,000 tons

Note the last number. Twenty five million tons of fusion isotope. If we use lithium 6 as the fuel isotope. We know that the fuel is a solid stiff metal. So we wont need any fuel tanks. We know that 7.42% of mined Lithium is the isotope we want (which is fortunate since we will need a lot of it.), and that the fuel isotope weights 462 Kilograms per cubic meter. So the 25 million tons, will take a tank with 54 million cubic meters of volume to hold it. If we use a cylindrical tank 200 meters in diameter with a central access shaft 50 meters in diameter. The resulting tank will be 1839 meters long. (We might want a wider shorter starship.)

The Explorer Class of starships is the oldest and the most detailed of the designs. Developed by the group. It was my design for a basic fusion powered starship with all the support structures food consumption, habitation deck, and the rest. Worked out in as much detail as we can manage. Its maximum speed is probably only about 1/3rd of light speed, and it can only get to that speed with the equipment to mine millions of tons of fusion isotopes, and a external fuel launcher system. So it wouldn't be fast enough to get to Tau Ceti in an acceptable amount of time (thou quite acceptable for a flight to Alpha Centuri or other near by stars). But it does seem to be a workable starship design, that doesn't require any technologies that don't seem possible within the next 50 years.

APPENDIX 1: Relativity

The time dilation effect (almost all other relativistic effects have a similar equation) is defined as:

t' = t/SQRT(1 - beta^2)

Where t' is the time aboard the ship and t is time to a relatively stationary observer (like Earth), or vice versa since it's a matter of reference whether the ship moves away from the Earth or the Earth moves away from the ship. beta is velocity in terms of light speed (v/c). This basically defines a curve that shoots up to infinity at a beta = 1.

Here are some values for the time dilation factor as a function of speed:

A book published by NASA in the mid '70's on space colonization. Basically, NASA's take on the old L-5 society's ideas. A light technical book written for the general public that covers all the basics from colony construction, space based industrial processes, launching lunar materials, mass flows within the life support / agricultural system, and psycological effects of different colony shapes. Lots of illustrations, graph, the works.